1. Field of the Invention
The present invention relates to a magnetic circuit and, more particularly, the present invention relates to a magnetic circuit suitably used for magnetic field heat treatment of a magnetic recording medium substrate and so on and a method of applying a magnetic field by means of the magnetic circuit.
2. Description of the Related Art
In the technical field of information recording, hard disk drives acting as means for magnetically reading and writing information including characters, images, and music have become necessary as primary external recorders or internal recording means of electronic equipment such as personal computers. Such hard disk drives include hard disks serving as magnetic recording media. For hard disks, a so-called “in-plane magnetic recording method (longitudinal magnetic recording method)” is available in which magnetic information is longitudinally written in the plane of a disk and “a perpendicular magnetic recording method” is available in which magnetic information is perpendicularly written in the plane of a disk.
FIG. 1 is a sectional view schematically showing a typical laminated structure of a hard disk of the longitudinal magnetic recording method. A non-magnetic Cr base layer 2 formed by a sputtering method, a magnetic recording layer 3, and a carbon layer 4 serving as a protective film are sequentially stacked on a non-magnetic substrate 1, and a liquid lubricating layer 5 formed by applying a lubricant is formed on a surface of the carbon layer 4. The layers are about 20 nm in thickness at the most and are generally formed by a dry process such as a magnetron sputtering method (for example, see Japanese Patent Laid-Open No. 5-143972). The magnetic recording layer 3 is made of a Co alloy having uniaxial crystal magnetic anisotropy. The Co alloy includes CoCrTa and CoCrPt alloys. The crystal grains of the Co alloy are horizontally magnetized relative to a surface of the disk to record information.
However, in the longitudinal magnetic recording method, an increase in recording density has been regarded as being limited because of the following problems: when each recording region (magnetic domain) is reduced in size to increase a recording density, the north poles and south poles of adjacent magnetic domains repel each other and result in cancellation of magnetization, so that for a high recording density, it is necessary to reduce the thickness of the magnetic recording layer and reduce the crystal grains in size, and finer crystal grains (smaller volumes) cause a phenomenon such as “thermal fluctuations” in which the magnetization direction of the crystal grains is disturbed by thermal energy and data is deleted.
In response to these problems, the “perpendicular magnetic recording method” has been studied. In this recording method, a magnetic recording layer is magnetized perpendicularly to a surface of a disk. Thus the north poles and the south poles are alternately combined and placed in a bit arrangement and the north poles and south poles of magnetic domains are adjacent to each other with enhanced magnetization. As a result, there is just a small number of self-demagnetizing fields (demagnetizing fields) in a bit and thus the magnetization (magnetic recording) is more stabilized. When the magnetization direction is recorded in the perpendicular direction, it is not necessary to significantly reduce the thickness of the magnetic recording layer. For this reason, even when the recording region is reduced in size in the horizontal direction, the recording layer is increased in thickness and the crystal grains are increased in size in the perpendicular direction, so that the overall crystal grains are increased in volume and the influence of “thermal fluctuations” can be reduced. Thus the perpendicular magnetic recording method is expected as a method for achieving super high density recording.
FIG. 2 is a sectional view schematically showing a basic layered structure of a hard disk acting as a “perpendicular two-layer magnetic recording medium” in which a recording layer for perpendicular magnetic recording is provided on a soft magnetic under layer (SUL). A soft magnetic under layer (SUL) 12, a magnetic recording layer 13, a protective layer 14, and a lubricating layer 15 are sequentially stacked on a non-magnetic substrate 11.
In this structure, the soft magnetic under layer 12 effectively acts to increase a writing magnetic field and reduce the demagnetizing field of the magnetic recording layer 13. Permalloy, CoZrNb amorphous, and so on are typically used for the soft magnetic under layer 12. For the magnetic recording layer 13, a CoCrPt alloy, a PtCo film, a PtFe film, or a SmCo amorphous film or the like is used.
As shown in FIG. 2, in a hard disk of the perpendicular two-layer magnetic recording method, the soft magnetic under layer 12 is provided as the base of the magnetic recording layer 13. The soft magnetic under layer 12 has a magnetic property of “soft magnetism” and has a thickness of about 100 nm to 500 nm. The soft magnetic under layer 12 is provided to increase a writing magnetic field and reduce the demagnetizing field of the magnetic recording film. Further, the soft magnetic under layer 12 acts as a path of a magnetic flux from the magnetic recording layer 13 and a path of a writing magnetic flux from a recording head. In other words, the soft magnetic under layer 12 plays the same role as an iron yoke provided in a permanent magnet magnetic circuit. Thus in order to avoid magnetic saturation during writing, the thickness of the soft magnetic under layer 12 has to be set larger than that of the magnetic recording layer 13.
In view of the multilayer configuration, the soft magnetic under layer 12 corresponds to the non-magnetic Cr base layer 2 provided in the hard disk of the longitudinal magnetic recording method shown in FIG. 1. However, the soft magnetic under layer 12 is not formed as easily as the Cr base layer 2.
As described above, in the hard disk of the longitudinal magnetic recording method, each layer is about 20 nm in thickness at the most and is formed by a dry process (mainly by magnetron sputtering, see Japanese Patent Laid-Open No. 5-143972). Also for perpendicular two-layer recording media, various methods have been studied to form the magnetic recording layer 13 and the soft magnetic under layer 12 by a dry process.
However, when the soft magnetic under layer 12 is formed by the dry process, a sputtering target has to be a ferromagnetic material having strong saturation magnetization and the soft magnetic under layer 12 has to be 100 nm or larger in thickness. For these reasons, perpendicular two-layer recording media have serious problems about mass production and productivity in consideration of the evenness of the film thickness and composition, the life of the target, the stability of the process, and the low deposition rate above all.
For this reason, attempts to apply a metal film on the non-magnetic substrate 11 by a plating method and use the metal film as the soft magnetic under layer 12 have been studied. In the plating method, the thickness of the metal film can be easily increased and can be polished.
FIG. 3 is an explanatory drawing showing a structural example of a perpendicular two-layer recording medium in which the soft magnetic under layer 12 is formed by plating. In this laminated structure, between the non-magnetic substrate 11 and the soft magnetic under layer 12, a nucleation film 16 for obtaining adhesion to the substrate is formed by plating.
Incidentally, a number of magnetic domains magnetized in a specific direction are prone to appear in the plane of a soft magnetic film and domain walls appear on the interfaces of the magnetic domains. When the soft magnetic film having such domain walls is used as a soft magnetic under layer for the perpendicular two-layer magnetic recording medium, a leakage magnetic field generated from the domain walls causes isolated pulse noise called spike noise or micro-spike noise, so that the signal reproduction property may be seriously deteriorated. As a solution to this problem, it is effective to make the soft magnetic film anisotropic in the in-plane radial direction serving as the easy axis of magnetization or in the in-plane circumferential direction.
FIG. 4 is an explanatory drawing of “magnetic anisotropy”. An anisotropic magnetic field (Hk) is provided as a difference (δH) between a magnetization saturation magnetic field strength in the in-plane radial direction and a magnetization saturation magnetic field strength in the in-plane circumferential direction. When δH is positive, the in-plane radial direction is the magnetization direction (anisotropy direction). When δH is negative, the in-plane circumferential direction is the magnetization direction (anisotropy direction). In this case, the numeric value of magnetic anisotropy is represented as an absolute value.
When the soft magnetic film is formed by a dry process (for example, a sputtering method), the soft magnetic film is provided with magnetic anisotropy in the in-plane radial direction by applying a radial magnetic field to the substrate during sputtering. When the soft magnetic film is formed by a wet process (for example, a plating method), the soft magnetic film can be provided with magnetic anisotropy in the in-plane circumferential direction by forming the soft magnetic film while applying a magnetic field in one direction to the substrate and rotating the substrate during plating. Such magnetic anisotropy is substantially axially symmetric with respect to the axis of the substrate and may be provided in either of the in-plane radial and circumferential directions according to the simulation results of a magnetic recording process.
However, in both of the dry and wet processes, it is not easy to simultaneously form a soft magnetic film with excellent film characteristics and provide the film with magnetic anisotropy having high axial symmetry. Thus a method for simultaneously achieving such excellent film characteristics and magnetic anisotropy is desired.
As an effective means of providing magnetic anisotropy, a technique of heat-treating a soft magnetic substance in a magnetic field is available. For example, in a fabrication process of a GMR head, heat treatment is performed in a strong magnetic field exceeding 1 tesla (T) to orient a pin layer and a free layer, so that magnetic anisotropy is provided in the direction of the magnetic field.
Further, it is known that in an audio-visual magnetic head, noise can be reduced by performing heat treatment in a rotating magnetic field (one of a magnetic field device and the head is rotated) to obtain magnetic isotropy.
However, in such a heat treatment process, it is necessary to form a heat treatment furnace including a non-magnetic component and generate a magnetic field in one direction in the heat treatment furnace. Thus a magnetic field generator tends to be large in size and require external installation and the heat treatment furnace tends to have a complicated configuration.
It can be said that a magnetic field heat treatment method for providing a soft magnetic substance with magnetic anisotropy or isotropy is an established technique. However, it is not easy to provide the soft magnetic under layer of the perpendicular two-layer recording medium with axially symmetric magnetic anisotropy in the in-plane radial direction and the in-plane circumferential direction through magnetic field heat treatment. This is because in the configuration for applying a magnetic field from the outside of the heat treatment furnace, it is difficult to form a magnetic field in the in-plane radial direction or the in-plane circumferential direction of the substrate disposed in the furnace.
A magnetic field in the in-plane radial direction can be relatively easily formed with coils having the same poles facing each other. However, a region enabling a diverging magnetic field in the in-plane radial direction suitably for obtaining magnetic anisotropy is small. Further, a strong magnetic field is hard to obtain.
Under these constraints, soft magnetic under layers have not been provided with axially symmetric magnetic anisotropy through magnetic field heat treatment.